THE AKMY IIOKSE. 39 



carotid and the two are separated by a thin muscle. The jugu- 

 hir veins in their descent follow grooves at the side of the neck 

 (jugular furrows), and at first are close to the surface and 

 easily felt ; they soon take a deeper course, running beneath the 

 panniculus carnosus muscle. The}^ enter the front part of the 

 thorax, where they empty into the anterior vena cava just in 

 front of the heart. 



The posterior vena cava is the main vein returning the blood 

 from the hind parts and from the abdominal and pelvic organs. 

 It corresponds to the posterior aorta, which, as has been seen, 

 is the main artery carrying the blood to these parts. This vein 

 is formed at the front part of the pelvis and runs forward 

 under the lumbar vertebra, accompanying the posterior aorta, 

 which is at its left. '\Alien it reaches the upper border of the 

 liver it inclines downward and passes through a notch or fis- 

 sure of that organ. Thence it passes through the diaphragm 

 into the thoracic cavity; here it follows a groove on the upper 

 surface of the right lung and then enters the right auricle of 

 the heart. 



The important veins of the foot will be discussed later. 



AXATOMY AXD PHYSIOLOGY OF THE LYMPHATIC SYSTEM!. 



The h'mphatic or absorbent system resembles the system of 

 blood veins with which it is connected. The main part of the 

 sj^stem collects surplus lymph (to be described later) and re- 

 turns it to the blood; a smaller part has the same function, but, 

 in addition, absorbs and collects chyle and adds it to the blood. 



Allien the blood in its circuit reaches the capillaries the 

 serum oozes through their thin walls into the minute spaces in 

 the surrounding tissues and there receives the name of lympli. 

 This colorless fluid bathes and nourishes the tissues and takes 

 up worn-out material. The spaces in the tissues assemble into 

 minute, delicate, and transparent vessels (lymphatics), which 

 are remarkable for their knotted appearance, due to numerous 

 valves. The vessels join and increase in size, like veins, and 

 through them flows the surplus lymph with its collected waste 

 material. 



The vessels of the right fore extremity, the right side of the 

 head, neck, and thorax, form tubes uniting in a main trunk, 

 called the right lymphatic vein., which leads into the anterior 

 vena cava ; the vessels from the remainder of the body unite in 

 a trunk "called the thoracic duct, which begins in the lumbar 



